I had giardia three times. The first time was the worst because it took a few months before it was correctly identified and treated-this was back in 1975. I took Flagyl for it, which I knew was something of a dangerous drug, but it knocked out the giardia. The next two times I got it, I recognized the exact symptoms, took Flagyl again and was done. I believe my physical weakness preceded this--in fact made me much more prone to this infection and getting it really badly, while others on the same camping trip didn't get sick, and others exposed to the same contaminated water the later times either didn't get sick or were only mildly ill and threw it off. But here is what happened in my case. It really shut down the small intestine (as a severe problem with gluten also does). Stools were pale, greasy and floated. Nausea was too bad to eat and eating just produced worse nausea and cramping. I got intense pain from eating bran cereal at some point, the first time I got it. Like an appendicitis bad. I lost weight so fast and became so fatigued that after awhile all I could do was stay in bed most of the time. I even got to the point of hallucinating, which I understand people with severe celiac disease can do too. I think I could have died from this first infection, due to starvation if nothing else. So, this was on the extreme reaction end of the scale and Flagyl restored my previous level of health right away.